Lyme disease is a multi-systemic disorder caused by Borrelia burgdorferi that spreads to mammals by the bite of an Ixodes tick. In the U.S. alone over 17,000 cases were reported in 2000, making it the number one reportable arthropod-borne infectious disease. In unfed, infected ticks the bacteria reside in the tick midgut, which has a pH/temperature of 8.0-8.4/23degreesC. During tick feeding and transmission the spirochetes experience a shift in pH/temperature from 8.0-8.4/23degreesC to 6.0- 7.4/35degreesC due to the influx of blood from the host. B. burgdorferi adapts to its new surroundings by altering the expression of different genes in response to environmental cues, such as changes in pH and temperature. We propose to delineate the molecular mechanisms that are involved in gene regulation in response to pH. Using affinity chromatography, mass spectroscopy and DNase I footprinting we propose to identify, characterize and inactivate the positive regulator of paralogous gene family 54 in B. burgdorferi. This gene family contains 13 genes that are located on linear plasmids. The role of these genes in the infection or adaptation process is unknown, but gene family members bba64, bba65, bba66, and bba73 are regulated by changes in pH and have been implicated in contributing to acute Lyme arthritis in model systems. By understanding how this bacterium is able to sense the environment and alter gene expression in response to changes in environmental cues, we hope to gain insight into i) the regulatory components necessary for adaptation and infection and ii) the role of pH regulation of target genes in mammalian pathogenesis. [unreadable] [unreadable]